A special steel widely used in many of the industries is sheet steel. Sheet steel is used in automobiles, appliances, and similar goods. It is manufactured in primary mills. Also scrap steel from vehicles, refrigerators and other sources is used in secondary mills with additional iron ore and coke to form more steel, including sheet steel.
It is a problem, especially in the primary mills, to produce a sheet steel free and clear of corrosion and stain. Customers are more and more demanding in that they desire efficiently produced, and clean sheet steel. This combination of requirements is extremely difficult to obtain. In the processing of sheet steel, any delay or slow down results in stain or other undesirable material being formed on the sheet steel.
It is an uneconomical and an inefficient use of resources to simply discard this steel or to forward it for reprocessing as has been done in the past. It is very desirable to process the steel and avoid the stain in the first place. Such a procedure for treating sheet steel is extremely difficult to carry out. Yet, in this process, difficulty is caused by the high speed processing and the reduced time for achieving these desired results. It is very desirable to develop a process for removing the stain from sheet steel during the course of manufacturing in a simplified efficient fashion.
Such a simplified, efficient process of removing stain from sheet steel is difficult to obtain. While many compositions are known to remove the stain from steel in other forms, it is extremely difficult to apply the compositions and remove the stain from sheet steel at high speed.
By achieving such removal at high speed, inefficiencies can result. Where such stain removal is achieved by the process of the prior art, it is done at a slower pace or with environmentally hazardous materials. It is very desirable to achieve this stain removal quickly, yet in an environmentally efficient fashion and avoid such stain reforming on the sheet steel. It is further desirable that the waste therefrom be environmentally neutral and easily disposable. Accordingly, if the development of a process or composition to accomplish these goals is achieved, great advantages are obtained both for environmental needs and other societal needs.
Hot rolled sheet steel processed on high speed coil lines (FIG. 1), when pickled with hydrochloric acid suffers from iron and chloride staining. Hot rolled sheet steel must be pickled to remove the scale that is formed during the hot rolling process. The scale is an oxide that is predominantly ferric oxide. However, the stains occur after the pickling tanks used in this sheet steel process, and in the subsequent rinse sections during line stops where residual iron and chloride ions react with and precipitate onto the sheet steel.
The composition of the stain varies with its proximity to the pickle tanks. Predominantly the stain comprises of ferrous and ferric hydroxides. There are also ferrous and ferric chlorides present. The percentage of ferrous and ferric chlorides present in the stain gradually increases as the stain nears the pickle section. While in the past, primary producers of sheet steel did not find these stains detrimental, these stains are now a major problem.
An increasing percentage of the hot rolled sheet steel is designated for commercial use. The commercial hot rolled sheet steel, which is stained, is usually sent to a service pickling company. This service company buys and pickles the hot rolled sheet steel before it is sold to the end users.
The service pickler buys the stained coils at a discounted price. Then, in turn, that service pickler sells the repickled coils at commercial prices. The service pickler does not experience the "staining" problem to the extent of the primary producer, because the service pickler runs the line at substantially lower line speeds.
The pricing differential between the primary producer's discounted price and the service pickler's commercial price causes the impetus for the primary producer to compete in this market. The widening price differential is greatly due to increases in costs of transporting and reprocessing of the stained coils. The primary producers need to find another way to produce this portion of the commerical grade hot rolled sheet steel within their own production facilities. Current technology does not allow for maximum capacity to produce a commercial grade hot rolled sheet steel. Producers must slow the high speed coil lines down from about 400 meters per minute to about 60 meters per minute to prevent the line stops, which create staining.
The line stops occur due to welding and cutting operations. The high speed coil lines are continuous coil lines. At the beginning of the process, the sheet steel coils are welded to the previous coil. At the end of the process, the steel coils are cut from the continuous steel strip. The high speed coil lines have sections to take up coil so that during welding and cutting operations there is enough sheet steel in the system to keep the strip moving through the pickling and rinse sections and to prevent stops. However at speeds of about 400 meters per minute there is not enough accumulated coil to prevent a line stop.
The user of proprietary chemicals attempts to minimize staining of the sheet steel. Chelating chemicals can form soluble complexes with iron ions in the rinse water, thus preventing the formation of iron precipitates. When the rinse water dilutes the residual acid film after pickling and its pH rises to about pH 6.0, hydrated iron oxides precipitate on the ferrous surface of the sheet steel. Wetting agents can aid in the removal of acid residues and minimize hydrochloric acid pickle carry over due to the wetting agents effect to produce thinner surface films on the sheet steel. The thinner films will contain less iron and chloride ions, which in turn reduces the extent of the stain formed during line stops.
The efficiency of the stain removing chemicals are directly related to rinsing techniques. Residual chloride ions contribute to the formation of the stain. The chloride ions are present due to the hydrochloric acid carry over. Rinsing is of major importance in order to remove these residual chloride ions. The rinsability of chloride ions are achieved by the method of rinsing (spray, immersion, or a combination of both), spray pressure and flow, and spray pattern Also, the use of squeegee rolls and their placement are critical for better rinsing. All of these rinsing techniques for high speed coil lines effect chloride removal. However, none of these techniques are completely effective at removing all of the chloride ions that help to create the stain.
Formulations designed to prevent iron and chloride staining are not effective due to pickle line configurations. This is especially true in areas of the pickle line before and after the squeegees, and the inaccessible areas of the sheet steel, which cannot be rinsed because of their proximity to the pickle tanks.
Known in the art is a method of removing iron- and copper- containing scale from the interior metal surface of a boiler utilizing an admixture of polycarboxylic acids and phosphonic acids and their salts thereof. Scale is also removed from steam boilers, petrochemical process equipment, feedwater heaters and associated piping, and in various types of pressure vessels, such as high pressure steam generating equipment utilized in electric power generation and other applications, in which water is circulated and heat transfer occurs. These metallic surfaces are internal parts of process equipment. Such treatments are used to restore the efficiency of the process equipment by removing the scale, as a maintenance procedure.
The characteristics of stain and scale also differ with regard to composition and creation Stain is the steel industry's jargon for hydrated ferrous and ferric oxides, and ferrous and ferric chlorides. The scale may also include cuprous and cupric oxides, and water-soluble salts such as calcium carbonate, calcium hydroxide.
The stain is produced on sheet steel in a relatively short time (less than 15 minutes) when the hydrochloric acid pickle line stops. During line stops the stain is formed from residual ferrous and ferric chlorides that remain on the sheet steel surface after hydrochloric acid pickling. When the rinse water dilutes the residual acid film left on the surface, and its pH rises to about pH 6.0, hydrated iron oxides can form and can be left on the surface as an insoluble precipitate. This flash rusting or staining may occur especially in the first rinse stage at low line speeds, or if the sheet steel dries out during line stops.
The formation of scale is only similar to the formation of stain in so much that both are formed due to the phenomenon of insolubility, such as is described in U.S. Pat. No. 4,666,528, incorporated herein by reference. The primary reason for scale formation is the fact that the solubilities of the scale-forming salts decrease with increases in temperature and concentrations. When feedwater is elevated to boiler water temperature and concentrations, the solubility of the scale forming salts is exceeded. Scale forms by one or a combination of the following mechanisms.
1) The improper precipitation of relatively insoluble feedwater hardness compounds (for example, calcium carbonate precipitating at the metal surface by the following possible mechanism): EQU 2 Ca(HCO.sub.3).sub.2 +Heat, yields, EQU 2 CaCO.sub.3 +2 H.sub.2 O+2 CO.sub.2.
2) The supersaturation, or crystallization, of relatively soluble dissolved solids (CaSO.sub.4,SiO.sub.2) in water contacting heat transfer surfaces. This undesired coating on the sheet steel is usually a mixture of calcium sulfate and silicon dioxide (CaSO.sub.4,SiO.sub.2).
3) The accumulation of corrosion products (iron and copper oxides) or other suspended matter in the feedwater with subsequent deposition on the high heat transfer metal surfaces.
This complex or group of complexes is very stable. Such stability makes it hard to dissolve or otherwise remove the complexes as desired.